The invention relates generally to steam turbines, and more particularly, to a self-contained shaft sealing system for a steam turbine.
The pressure boundary of a steam turbine casing is penetrated by a rotating turbine shaft in order to transmit power generated by the turbine outside of the steam environment. Consequently, the shaft must be sealed at the points where it penetrates the casing in order to prevent steam from escaping, which may be dangerous to individuals in the area. The shaft seals must also prevent air from entering the casing, which would have detrimental effects on turbine performance.
A variety of shaft sealing systems have been employed including, for example, labyrinth seals disposed about the shaft ends. Labyrinth seals include teeth which enclose but do not contact the shaft, thus forming leakage paths between the seal and the shaft. Shaft sealing systems further include air seals, which function mainly to prevent air from entering the steam turbine. Disposed axially inward of the air seals are steam seals, which prevent steam from escaping to the outside of the steam turbine. To maintain a positive pressure difference across the air seal and the steam seal, accessories including piping systems, steam seal regulators, gland condensers, and auxiliary boilers are required to support turbine function. In some cases, a brush seal is used with labyrinth seals to reduce leakage, but the foregoing accessories remain necessary to provide proper turbine function. The gland condenser is employed to maintain a slight vacuum to draw out air that has passed the air seal in the direction of entering the casing, and to exhaust out steam that has passed the steam seal in the direction of exiting the casing. A seal header is typically maintained at a positive pressure and either supplies steam to the annulus inward of the steam seal or dumps steam from the annulus, as required in dependence upon whether steam leakage across the steam seal is exceeds or is exceeded by the leakage out of the internal section of the steam turbine. The positive pressure at the annulus inward of the steam seal precludes entry of air into the turbine.
As noted, labyrinth shaft sealing systems such as those described require the support of an extensive accessory system which does not contribute to the work performed by the turbine. These features increase the footprint of a plant, as well as the maintenance requirements without making any direct contribution to turbine output.
One alternative to labyrinth seals or brush seals is the use of carbon segmented circumferential seals or face seals, which have smaller effective clearances than the typical 0.75 mm to 1 mm clearances found in labyrinth seal designs. The clearances of labyrinth seals allow for particulate matter in the ambient air to pass through the seals without a problem. In seal system designs that utilize carbon seals having much smaller clearances, however, particulate matter may become trapped in the clearance space, causing seal damage. This presents a challenge, particularly for plants such as, for example, coal-fired plants where particulate matter such as coal dust is common in ambient air.